Imaginary Power, Anyone?

I've been reviewing several technologies Tennessee Valley Authority (TVA) is presently investigating for use on its transmission system. Having personally spent 22 years in research, I'm not easy to impress. But the technical staff at TVA managed to wow me with several promising devices coming out of universities and tech start-ups.

In August, I traveled to TVA headquarters in Knoxville, Tennessee, to meet with Dale Bradshaw, manager of power delivery technologies. Bradshaw invited me to participate with 20 engineers and managers from several departments, including research, planning, operations, engineering and generation, in a day-long course on synchronous condensers.

Although I've heard of synchronous condensers, I have never investigated them at any great depth. Jerry Heydt, a regents' professor with Arizona State University, taught us the basics on how these devices generate volt amp reactive (VAR). Fortunately, Heydt has a gift for making technology seem simple.

Most of us are already familiar with two synchronous machines: the ac motor and the ac generator. Both devices contain rotors and stators with the inductance of the stator winding depending on the rotor position. Energy can be extracted from the magnetic field either as mechanical energy (a motor) or electrical energy (a generator).

Synchronous condensers are rotating machines that operate like motors and generators. But the goal is to create reactive power measured in VAR. The device operates in a borderline condition between a motor and a generator. This results in the delivery of reactive power, as the current is 90 degrees out of phase with voltage.

Synchronous condensers have been around for a while, but they are fairly high-maintenance devices with moving parts and significant mechanical and thermal stresses. Still, with our grid system getting more brittle, they need more fast, dynamic sources of reactive power. With the proliferation of independent generators, who are rewarded only for delivering real power, utilities increasingly need the ability to inject VAR into the network in seconds to avoid the potential of a fast voltage collapse.

Bradshaw, along with Terry Boston, TVA's executive vice president of transmission and power supply, encouraged American Superconductor (AMSC; Westborough, Massachusetts, U.S.) to consider developing a superconducting synchronous condenser. AMSC went to the drawing board and came up with a design that houses a superconducting rotor with a traditionally designed rotor and shaft. This new design cuts the losses in half and improves the transient response of the device dramatically over conventional synchronous condensers.

AMSC's Dr. Swarn Kalsi explained to the class how the company's prototype device, the SuperVAR, works. This device is designed to output 8 MVAR under steady-state conditions. The rotor is wound with thin ribbons that contain superconducting filaments placed in a silver-alloy matrix. The rotor is cooled using liquid neon, which is cooled to 30 degrees Kelvin by gaseous helium refrigerated with a cryogenic cooler. The coolers are actually fairly well along on the technology curve with a mean-time-between-failure (MTBF) of nine years. Yearly maintenance includes changing out the various filters and replacing the bearing oil; therefore, maintenance shouldn't be too onerous.

The first 8 MVAR SuperVAR will be installed on a TVA substation near Gallatin, Tennessee, to control power fluctuations coming from a nearby arc furnace. TVA ultimately intends to install these devices on the distribution side of major substations, which will enable them to inject VAR back onto the transmission grid.

The SuperVAR can be used as a shunt reactor or shunt capacitor, but the real value in the SuperVAR is its instantaneous capacity. The device can provide VAR at seven times nominal rating for five to six cycles to address disturbances caused by system faults. It also can provide VAR at two times rating for several minutes, giving it significant reserve capacity at a price competitive with current technology, even while using first generation superconducting wire. With mass production and the use of second generation Yttrium Boron Copper Oxide (YBCO) wire, the economics will improve to make this technology a clear winner in the value proposition of providing fast sources of reactive compensation.

I'm impressed that TVA has maintained its commitment to technology. Bradshaw and his group have been given the freedom to investigate this and other innovations, which are working their way through the pipeline and promise to further stabilize the grid or to increase system capacity.

The technologies TVA and other utilities are developing can only help us as they provide us with more tools to do our job. Given the appropriate resources, utility staffs are more than capable of building and maintaining a reliable grid. With ingenuity, we can even deliver “imaginary power.”